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IB DP Physics Mock Exam HL Paper 1B Set 5 - 2025 Syllabus

IB DP Physics Mock Exam HL Paper 1B Set 5

Prepare for the IB DP Physics Mock Exam HL Paper 1B Set 5 with our comprehensive mock exam set 5. Test your knowledge and understanding of key concepts with challenging questions covering all essential topics. Identify areas for improvement and boost your confidence for the real exam

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Question 

Students arrange a double-slit experiment in order to find the wavelength of monochromatic light. Light is directed onto a pair of slits with an adjustable separation \(d\). An interference pattern appears on a screen positioned at a fixed distance from the slits. The spacing between adjacent fringes is denoted by \(s\).
(a) A ruler marked in millimetres is used to measure \(s\). Describe a procedure for measuring \(s\) that reduces its absolute uncertainty.
(b) Explain how using a smaller value of \(d\) affects the fractional uncertainty in the measurement of \(s\).
(c) (i) To calculate \(\lambda\), students plot a graph with \(s\) on the vertical axis. State which quantity must be plotted on the horizontal axis so that the best-fit line is straight.
(ii) From the graph, a calculator gives \(\lambda = 611.089\,nm\) with a percentage uncertainty of \(\pm 5\%\). State how this result should be written in a final conclusion, including the absolute uncertainty and using an appropriate number of significant figures.
▶️ Answer/Explanation
Detailed solution

(a)
To reduce absolute uncertainty, measure the total distance across multiple fringes (for example, 10 fringes) and divide this distance by the number of fringe spacings.

(b)
From the relation \(s = \frac{\lambda D}{d}\), using a smaller slit separation \(d\) produces a larger fringe spacing \(s\). Since the absolute uncertainty of the ruler is unchanged, the fractional uncertainty \(\frac{\Delta s}{s}\) becomes smaller.

(c)
(i) As \(s = (\lambda D)\frac{1}{d}\), plotting \(s\) against \(\frac{1}{d}\) gives a straight-line graph through the origin.
(ii) The absolute uncertainty is \(5\%\) of \(611.089\,nm\), which is approximately \(30\,nm\). Rounding the value to match this uncertainty gives:
\(\lambda = 610 \pm 30\,nm\).

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